Sharks are among the oldest living things .
Approximately 200 to 250 species of sharks
live in all the oceans of the world.
Despite the shark's reputation for viciousness,
only a few species of sharks are known to attack humans.
Description and Habits
Sharks have skeletons composed of cartilage rather than of bone.
With a few exceptions, sharks have torpedo-shaped bodies an efficient,
streamlined design for fast-swimming predators.
Bottom dwellers tend to be stout and heavy bodied.
Angel sharks are flat, like the rays .
The hammerhead shark has a flattened head that
resembles a double-headed hammer, with an eye on each stalk.
The whale shark is the largest of the sharks
more than 45 feet long and the 6-inch dwarf shark is the smallest.
Sharks may be reddish brown, bronze, metallic blue,
gray, or nearly black and are often patterned with
spots, bands, marblings, or protuberances.
Cat sharks are colorful and exquisitely patterned.
A shark's tough skin is embedded with enamel-covered
dermal denticles sharp toothlike structures.
The pointed end of these denticles projects toward the tail.
These modified scales may overlap or may be widely spaced.
A shark has three types of unpaired fins:
one or two dorsal fins on its back,
an anal fin beneath its body (not present in all species),
and a caudal fin the tail itself.
There are two sets of paired fins:
the pectoral fins in front,
which are used as steering rudders,
and the pelvic fins at the rear.
Sharks are known for their speed and
maneuverability in the water.
Most species can swim at speeds of 20 to 30 miles per hour,
and the speed of the mako has been recorded
at more than 60 miles per hour.
Sharks have five to seven gill slits.
Many sharks must keep moving in order to breathe
that is, in order to keep water moving past
their gill slits but this is not true of all species.
The shark has senses specially adapted for
the animal's life underwater.
Smell is a shark's most acute sense.
Experiments have shown that sharks can detect prey
solely by smell, and the hungrier the shark,
the less stimulant needed to elicit a reaction.
Sharks' diets vary considerably and may include
plankton, clams, crabs, sea turtles, fishes,
seals, porpoises, and whales.
Whale sharks and basking sharks strain plankton
from the sea through modified gill rakers.
The thresher shark uses its long tail,
which constitutes half its body length, to thrash
and stun schooling fishes feeding near the surface.
Reproduction
Incubation takes from 6 to 15 months
depending on the species.Gestation periods vary,
but may last as long as two years.
The young, called pups, are born fully
developed and independent. The usual litter size
is 2 to 20, but the number of pups may exceed 100.
Behavior
Sharks may bite when provoked, but fewer than 30 species
are considered dangerous, regardless of the situation.
The two largest species, the whale and basking sharks,
are harmless plankton feeders. Sharks will attack humans
at any time of day, in warm or cold water.
Although most attacks are recorded during
daylight hours in shallow warm waters accessible
from a public beach, these statistics may simply
reflect the fact that these are the conditions in
which the greatest numbers of swimmers are found.
The waters of coastal North America, South Africa,
and the Caribbean and Mediterranean seas are the most
frequent sites of shark attacks.
Large sharks, such as the great white, tiger,
and bull sharks, that include human-sized prey
in their diet are the most dangerous.
Hammerhead, gray reef, lemon, dusky, blue, spinner,
sand tiger, nurse, and Ganges River sharks
will also attack humans.Sharks will attack when
they are hungry, but in most cases the reason
for attack is unknown. Possible causes include
territorial defense, mistaken identity for some
other form of prey.This might explain why a shark
often ceases its attack after one bite,
chemical attractants such as blood in the water,
and simply the movement, noises, and splashing of swimmers.
